Controlling where shards are allocated

Question

My setup:

• two zoness: fast and slow with 5 nodes each.
• fast nodes have ephemeral storage, whereas the slow nodes are NFS based.
• Running Elasticsearch OSS v7.7.1. (I have no control over the version)
• I have the following cluster setting: cluster.routing.allocation.awareness.attributes: zone
• My index has 2 replicas, so 3 shard instances (1x primary, 2x replica)

I am trying to ensure the following:

• 1 of the 3 shard instances to be located in zone fast.
• 2 of the 3 shard instances to be located in zone slow (because it has persistent storage)
• Queries to be run in shard in zone fast where available.
• Inserts to only return as written once its written once its been replicated.

Is this setup possible?

Link to a related question: How do I control where my primary and replica shards are located?

EDIT to add extra information:

Both fast and slow nodes run on a PaaS offering where we are not in control of hardware restarts meaning there can technically be non-graceful shutdowns/restarts at any point.

I'm worried about unflushed data and/or index corruption so I am looking for multiple replicas to be on the slow zone nodes backed by NFS to reduce the likelihood of data loss, despite the fact that this will "overload" the slow zone with redundant data.